Auto Guide is reporting that Gas2.org is reporting that the 2015 Mustang body-in-white will be offered with a solid rear axle. That is all the news I have.
As for speculation, I imagine that FRPP will offer the conversion kit. But how will it be mounted? S197-style 3-link? SN95-style 4-link? Parallel 4-link with Panhard? Torque arm and Panhard? My vote goes for the 3-link with Panhard.